Production of an industrial product such as a biodiesel fuel and a food product (hereinafter, referred to as “a biofuel or the like”) from a fatty acid produced by a unicellular photosynthetic organism (hereinafter, referred to as a “microalga”) or from a compound (hereinafter, referred to as a “lipid”) which releases a fatty acid by hydrolysis has been studied intensively all over the world. The inventors of the present application have found the following with respect to the lipid production.
At present, the production of a biofuel or the like on a commercial basis may be difficult due to high production cost. Thus, development of a technique reducing the production cost of a biofuel or the like may be necessary (for example, Non-Patent Literature 1).
In an attempt to isolate a microalga with increased lipid productivity and thus to contribute to the cost reduction, some studies have already been conducted. In general, a photosynthetic organism has excessive antenna chlorophyll, which reduces the efficiency for light utilization. Therefore, it has been confirmed that the biomass productivity is improved by reducing the antenna chlorophyll content (Non-Patent Literatures 2 to 4).
In addition, with the intention of reducing labor for lipid recovery, strains which secrete lipids into extracellular spaces have been produced. However, the strains have not been put to practical use (Non-Patent Literatures 5 and 6).
Compared to the enormous interest in the biofuel production using a microalga and compared to the high number of reviews of biofuel processes using microalgal biomass and breeding of microalgae, a very small number of strains have been developed, as described above.
Strain Pseudochoricystis ellipsoidea (P. ellipsoidea) has been reported by Satoh et al. (Non-Patent Literature 7). The genus and species names of this strain are not according to the International Code of Nomenclature for algae, fungi, and plants but are tentative names. The subsequent phylogenetic analysis using genes has demonstrated that the strain is a close relative of Coccomyxa and Pseudococcomyxa. The strain, like other microalgae, accumulates lipids in the cell when the nitrogen source in the culture medium is depleted (Non-Patent Literature 8). The mechanism of the lipid accumulation induced by the depletion of the nitrogen source has not been revealed yet, but it is speculated that lipids are accumulated as a result of the cell response to the stress of nitrogen depletion (Non-Patent Literature 9).
The inventors of the present application have been working on breeding P. ellipsoidea and improving large-scale cultivation techniques. For example, as described in Patent Literature 1, the inventors have succeeded in culturing two P. ellipsoidea strains (strain Obi and strain N1: Patent Literature 2) in an outdoor open system for a long time.
Similar to general photosynthetic organisms, P. ellipsoidea also has excessive antenna chlorophyll, which reduces the efficiency for light utilization. Patent Literature 3 discloses that strain 5P with a reduced antenna chlorophyll content was isolated using P. ellipsoidea strain Obi as the parental strain. The biomass productivity of strain 5P was superior to that of the wild-type strain (Patent Literature 3).
P. ellipsoidea is a strain suitable for outdoor cultivation on a large scale and may be considered as one of the most promising strains as the raw materials for the commercial lipid production. Further improvement of the lipid productivity and the reduction in the lipid production cost are desired.